The coefficient of friction is the ratio of the friction force to the normal force. There are several known methods to measure the coefficient of friction. The Horizontal Pull Slipmeter (“HPS”) consists of two basic elements: (1) a drag sled mounted on slider feet and equipped with a force gauge, and (2) a device for pulling the sled at a controlled rate of speed across the surface being tested. Another device, described in U.S. Pat. No. 4,895,015, is an improvement to the HPS that utilizes a drag sled and a stationary pulling mechanism with a set of guide tracks. The drag sled is pulled by a motor and guided along the tracks, which assure alignment of the pull. The force required to pull the drag sled is measured, thus allowing a determination of the coefficient of friction.
Another device, described in U.S. Pat. No. 4,813,266, consists of a test block connected to a microprocessor. The bottom of the test block is covered with a material for which the coefficient of friction will be measured. The test block is set on a test surface and the block is given an impulse with a hammer or other object. The impulse causes the block to move along the surface. An accelerometer on the test block transmits acceleration data to the microprocessor. The acceleration data includes information on the acceleration of the block caused by the impulse and the deceleration of the block as it comes to a stop after the impulse. This acceleration data is used by the microprocessor to determine the coefficient of friction at a specific point on the test surface.
In laboratories, tire testing machines typically use an expendable material such as 80-grit sandpaper to simulate a roadway surface. As tests are conducted, the aggregate of the sandpaper begins to wear and degrade, resulting in a less abrasive simulated roadway surface. This can jeopardize the test data.
In most cases it is desirable, but impractical, to maintain a constant coefficient of friction between the tire and the roadway surface. To prevent testing of a tire on an overly worn or inconsistent surface, the laboratory operator must monitor surface degradation of the sandpaper. It is known to monitor surface degradation visually or by running a hand over the width of the sandpaper surface, feeling for a change in surface roughness. When significant variation is felt across various points on the surface, the sandpaper is considered “worn,” and should be replaced. New sandpaper is applied for further equipment testing.
This method of monitoring sandpaper surface wear can be problematic because it is based on a subjective evaluation of the sandpaper surface roughness, which is not sufficient because it is highly operator-dependant and can result in two error modes: (1) prematurely changing the sandpaper, which is labor intensive and increases costs; or (2) conducting sensitive tire testing on a non-representative (worn) sandpaper surface. A sandpaper surface can, for example, cost $200 and removal/replacement can take 6-8 hours of technician time. Data collected for a worn sandpaper surface can jeopardize computer models.
A quantitative method is needed in which the coefficient of friction is measured and tracked for improve data integrity.